In mammals and the fruit fly, the vast array of odors in the
environment is discriminated by a large number of receptor molecules [1,
2, 3]. Individual olfactory sensory neurons express only one of the many
receptor genes [1, 2, 3]. Neurons expressing the same receptor gene
project to the same glomerulus [4, 5, 6], providing the anatomical
evidence for a spatial coding mechanism. Electrophysiological recordings
from olfactory neurons suggest that the temporal pattern of their
responses can also convey information about odor quality [7].
Odor-induced oscillatory activity, an indication of synchrony, has been
observed in phylogenetically different species, including molluscs,
insects, and mammals [7, 8, 9, 10, 11, 12].

The adult Drosophila antennal lobe, organized in spheroidal
subcompartments termed glomeruli, receives about 1200 olfactory
afferents from the antenna and 120 afferent fibers from the maxillary
palp [13]. Although the fly and mammals share the similarity that
receptor neurons expressing the same receptor gene project to one or two
glomeruli in a stereotypic manner [4, 5, 6], there are only 60 receptor
genes and 43 glomeruli in Drosophila, in contrast to the 1000 receptor
genes and 1800 glomeruli within the olfactory bulb of mammals [1, 2, 3].
The lower complexity in anatomy and the rich behavioral repertoire in
Drosophila makes it an attractive system with which to study olfaction.
Moreover, sophisticated genetic tools and behavioral mutants can now
also be used to study the olfactory system in Drosophila. Nevertheless,
understanding mechanisms of odor discrimination in the CNS of the fly
has been difficult due to a lack of physiological tools for functional
studies.

Odor-induced oscillations have been observed in several insect
species, including the locust, cockroach, honeybee, bumblebee, and wasp
[7]. Local field potential LFP recordings show odor-induced oscillation
at [sim]10 Hz which typically lasts for the duration of odor
stimulation. I have investigated this phenomenon in the Drosophila CNS.
LFPs were recorded with glass electrodes (tip, 5 [micro]m) that were
filled with Drosophila HL3 saline and positioned with a motorized
manipulator (MP285, Sutter). A patch clamp amplifier (EPC 7, Heka) was
used, and the signal was filtered (band pass at 0.1 to 20 Hz) with a
signal conditioner (CyberAmp, Axon Instruments) and recorded with
software (AxoScope, Axon Instruments) run on a PC. Adult flies (less
than a week after eclosion) were lightly anesthetized with [CO.sub.2]
and decapitated. The heads were immobilized with wax on a microscope
slide with the antennae pointing upward. A small opening was made on the
dorsal cuticle for the extracellular recording.

Figure 1 shows LFP recordings from the CNS of the Canton-S
wild-type fly that reveal an odor-induced oscillation. This phenomenon
was confirmed in 6 preparations. A power spectrum analysis indicates
that the major frequency components are less than 4 Hz (Fig. 1). This
LFP oscillation signal appears to be sensitive to the position of the
electrode, and the coordinates taken from the manipulator suggest that
the recordings may have originated in the antennal lobe. Future
experiments with GFP-labeled antennal lobe may help in identifying the
sources of the oscillatory activity. The patterns of oscillation in
response to the same odor appear to be roughly similar in sequential
recordings from the same animal. The LFP patterns generated in response
to peppermint (from McCormick) and amyl acetate (from Sigma) were
distinguishable by eye. Moreover, the power spectrum analysis indicates
that peppermint generates slightly more high frequency components.

This is the first LFP recording from the Drosophila CNS. The
preliminary results presented here show that odor-induced oscillation
occurs in Drosophila; this finding suggests that a temporal coding
mechanism may be employed by the fly, and that the power of genetics may
be applied in the future to decipher the physiological significance of
the odor-induced oscillation.

I would like to thank Alan Gelperin for his generous support,
Leonardo Belluscio for critical comments on the manuscript, and Carl
Zeiss, Inc., and Axon Instruments, Inc., for providing equipment. This
research was carried out in the Grass Laboratory at the Marine
Biological Laboratory, Woods Hole, Massachusetts, and was supported by
the Grass Foundation.